Parent Vessel Size and Curvature Strongly Influence Risk of Incomplete Stent Apposition in Enterprise Intracranial Aneurysm Stent Coiling

Discussion

ISA has been defined by Rathore et al⁸ as the “separation of one or more stent struts from the underlying vessel wall and observation of blood speckles between the stent struts and the vessel wall,” as demonstrated with the use of intravascular sonography for visualization of the stent-containing vessel. By confirming our findings by using FPCT imaging, which has been shown to be effective in the visualization of in vivo stents,^13,14 we demonstrated the same blood flow phenomenon between the stent struts and the vessel wall by using the presence of the CS on 3T-MRA imaging. Univariate and multivariate results from this study demonstrate that following deployment of closed-cell design ENs in the intracranial ICA, ISA is more likely to be found in vessels with more tortuous anatomy.

Prior studies evaluating ISA in balloon-mounted stents deployed for coronary atherosclerosis have identified predictors of ISA to include atherosclerotic lesion length,¹⁵ use of drug-eluting stents, unstable angina, and the absence of diabetes.¹⁶ The significant differences in properties between balloon-mounted coronary stents and the low-radial-force self-expanding ENs make ISA etiology comparison between the 2 less compelling. EN consists of a self-expanding closed-cell design, which is significantly less stiff and has thinner struts than balloon-mounted variants.

The clinical impact of ISA in the coronary vasculature remains under debate. Onizuka et al¹⁷ demonstrated that no adverse clinical events occurred in 15 patients found to harbor ISA following carotid artery angioplasty and stent placement with a mean follow-up of 11 months. The RAVEL trial¹⁸ demonstrated no adverse events in patients found to harbor ISA with a follow-up of 12 months. Other studies evaluating the long-term consequences of ISA have highlighted a significant risk. Hoffmann et al¹⁹ described a higher incidence of myocardial infarction in patients with ISA after 4 years of follow-up compared with patients without ISA. Siqueira et al¹⁰ performed follow-up on patients for a mean of 29 months and observed an increased incidence of stent thrombosis in patients with ISA. Cook et al²⁰ reported similar results, finding that patients presenting with stent thrombosis after a minimum of 12 months following stent deployment had a higher incidence of ISA. In their review of the coronary literature and the aforementioned studies, Rathore et al⁸ concluded that a definite relationship existed between ISA and stent thrombosis, though they conceded that longer follow-up is necessary to elucidate the true consequences of ISA. Because the coronary stents evaluated in these previous studies are balloon-mounted and constructed of thicker and stiffer stent struts, the results elucidated in these studies in patients with coronary atherosclerosis cannot be easily correlated to stent-mediated aneurysm coiling by using self-expanding flexible stents deployed in healthier nonstenosed vessel segments.

The finding of prevalent ISA in tightly curved vessels may be a direct result of the closed-cell design of the EN. Although the closed-cell design is advantageous at serving as a smooth interface at the aneurysm neck to act as scaffolding for coil deployment, our results point to this design as an inherent limitation in tortuous vessels having small curvature radius. In addition, the higher propensity for the EN to demonstrate ISA in larger diameter vessels suggests that the stent lacks outer radial force to sufficiently appose the wall near its deployment diameter. The EN may be better suited for use in smaller caliber vessels if significant curvature exists. If the vessel diameter is close to 4.0 mm, then the EN may be better limited to straight-vessel segments because its use in larger curved vessels could be associated with ISA. The current EN is manufactured in varying lengths but at a single uniform maximum deployed diameter of 4.5 mm. The findings presented here suggest the need for a larger EN configuration for use in vessels of near 4-mm luminal diameter. The findings suggest that 1 way to decrease ISA likelihood in ICA EN coiling may be to select a landing zone for the stent deployment that attempts to decrease the stent-subtended angle at any given aneurysm location as suggested in Fig 3B (left panel instead of right), provided that this is possible without compromising the ability of the stent to mechanically scaffold the coils properly.

The presence of poor stent apposition with the EN also has at least 2 technical ramifications for subsequent endovascular intervention. In the first, the presence of struts in the center of the vessel lumen may interfere with future microcatheter navigation or the addition of concentric stent placement; worse, it may lead to stent movement during microcatheter manipulation. In the second, the lack of stent apposition to the vessel wall harboring the aneurysm neck could potentially result in unintended partial coil herniation into the parent vessel or at least a higher exposure of the coil pack to intraluminal flow.

The EN has been linked by a number of case reports to a delayed migration phenomenon in the posterior circulation, which remains poorly explained.^21–23 The poor stent apposition highlighted in curved-vessel segments adds another possible mechanism of stent migration, because the lack of stent strut contact to the vessel wall may make the already lubricious coated stent less anchored and more prone to sliding. Nonetheless, the current study is predominantly in the anterior circulation and may, therefore, be of limited relevance to the cases of migration, which were in the posterior circulation. ISA, however, also increases the surface area of the stent within the vessel lumen, thereby exposing the stent to the central high-flow velocities expected in the approximately parabolic velocity profile in the target vessels. Although such greater exposure would increase the hemodynamic drag on the stent, it is not likely to explain the reported cases of migration, which led to proximal, not distal, delayed movement of the deployed EN.

Etiologically, the crescent-shaped 3T time-of-flight MR imaging signal intensity in tightly curved vessels with ISA may be directly related to the curved nature of the artery and the less curved nature of the closed-cell design EN. Measurements of the volume of the CS by using summation of the 3T-MRA signal intensity as a potential proportional indicator of ISA severity did not demonstrate a linear relationship to the morphologic features reported in the study (data not shown). It is likely that the 3T-MRA CS is not linearly proportional to the degree of ISA and may be best used as a binary indicator. This is possibly the result of the relatively low spatial resolution of MR imaging and of metal artifact signal-intensity dropoff from the stent struts. Both could lead to the inability of 3T-MRA to sufficiently delineate the small margins of the CS area. A comparison of the crescentic area of ISA in Figs 1 and 2 between 3T-MRA CS and the corresponding FPCT images also confirms the lower spatial resolution of 3T-MRA. These limitations of MR imaging suggest that the true degree of less severe ISA may be underestimated and may be even higher than that reported in the current study. A more detailed quantitative analysis of the area and shape of the ISA would likely necessitate the use of FPCT for computation of the orphaned volume in the zone of ISA.

In light of the link between vessel anatomy, relative stent-deployment location, and development of ISA in the ICA, we recommend that use of closed-cell design ENs be reserved for vessels with narrower diameters and larger radii of curvature. Preferential selection of these vessels may promote a lower likelihood of stent malapposition and a more favorable deployed-stent architecture. Given the significant lack of stent-vessel wall apposition at vessel diameters close to 4.0 mm, stent-subtended arc angles >100°, and tight vessel curvature, procedures requiring stent deployment in tortuous intracranial locations such as the carotid siphon warrant careful analysis. The current findings may also be relevant to recent attempts to employ the EN for the treatment of intracranial atherosclerosis because these suggest that it may be better suited for straight rather than curved segments.²⁴